A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to impart a beam of radiation with a pattern in its cross-section, the pattern corresponding to circuit pattern of an individual layer of the IC. This pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing or projecting an image of the entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
A lithographic printing process typically includes pre-exposure processes applied to the substrate prior to the imaging step, such as priming, resist coating and a soft baking of the substrate. After the imaging step, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to provide printed features constituting a pattern of an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, all intended to finish off an individual layer.
Typically, the aforementioned imaging or projection of the pattern onto the substrate is performed by having the patterned beam of radiation traverse a projection system. The projection system may comprise a series of lenses, and is arranged to project the pattern with high precision, i.e. introducing only small amounts of distortion or other errors into the projected pattern. In conventional operation of a lithographic apparatus, a multiplicity of substrates are patterned in series, one after another. Over time the projection system will heat up, due to absorption of the radiation which passes through it during projection of the pattern. This heating causes the shapes of the lenses and/or other optical elements of the projection system to change, thereby causing optical aberrations to occur which distort the pattern projected onto the substrate, or which or otherwise affect the fidelity of the pattern imaged on the substrate. Consequently, the pattern fidelity of imaged, as well as of printed, features may be affected over time, due to such heating.
The optical aberrations may be corrected by using actuators to adjust, for example, a shape of one or more of the lenses. However, this correction only works to a limited extent. There is a tendency in lithography towards using illumination modes in which radiation is increasingly concentrated over smaller areas during projection by the projection system. A lithographic apparatus generally comprises an illumination system which receives radiation from a source, such as a laser or an EUV radiation source, and produces the aforementioned beam of radiation for illuminating the patterning device. Within a typical illumination system, the beam is shaped and controlled such that at a pupil plane of the illumination system the beam has a desired spatial intensity distribution. The spatial intensity distribution at the pupil plane effectively acts as a virtual radiation source for providing illumination radiation at patterning device level. Any specific shape of the intensity distribution may be referred to as an illumination mode. Illumination modes used for printing patterns are, for example, “conventional illumination” (a top-hat disc-shaped intensity distribution in the pupil), and “off-axis” illumination modes such as annular, dipole, quadrupole and more complex shaped arrangements of the illumination pupil intensity distribution. The above mentioned tendency concerns in particular with off-axis illumination modes, wherein radiation impinging on the patterning device at an off-axis direction is increasingly concentrated over smaller areas in the pupil of the illumination system and the projection system during substrate exposure. This concentration of the radiation increases the extent to which optical elements of the projection system are heated or locally heated by the radiation. This in turn increases a distortion or blurring of the projected image and may decrease the fidelity or quality of the printed pattern beyond tolerance.